1. Field of the Invention
This invention resides in the field of rocket engine design, with particular applicability to heat exchange structures for rocket chambers and nozzles.
2. Description of the Prior Art
Rocket engines generally require a durable construction that can withstand extreme conditions of temperature and pressure encountered during both takeoff and flight. Extreme conditions are also encountered by other types of equipment, such as hypersonic vehicle surfaces, missile nose tips, radar domes, and the like. Using rocket engines as examples, those for which structural durability is most critical are supersonic engines, ramjets, and SCRAMJETs (supersonic combustion ramjets).
Durability is achieved in these structures by cooling the surfaces of components where high heat is generated. One means of cooling that is used in rocket engines is regenerative cooling, so-called because it draws some of the waste heat energy generated by the engine and recycles it back to the engine to produce a higher thrust. One type of engine in which regenerative cooling is used is an expander cycle rocket engine. In this engine, the internal walls of the combustion chamber and nozzle are cooled by uncombusted fuel fed by a turbopump into a jacket that surrounds the chamber and nozzle. This uncombusted fuel, now heated, is fed directly to the combustion chamber for combustion. The turbopump itself is driven by the heated fuel emerging from the jacket, and the same heated fuel also drives a second turbopump supplying oxidizer to the combustion chamber. A single turbine is often used to drive both turbopumps. The fuel emerging from the turbine and the oxidizer that is pumped by the oxidizer turbopump are both directed to the engine combustion chamber. The expander cycle thus cools the chamber while using heat from the chamber to increase the flow rates of both fuel and oxidizer to the chamber and thereby raise the chamber pressure. Since some of the heat is retained by the fuel, the cycle also serves as a means of preheating the fuel. Coolant jackets are also used in rocket engines that are not of the expander cycle type. Water, for example, is used as a coolant in rocket-based combined cycle (RBCC) engines that are not designed for flight at all.
The coolant jackets typically used in the combustion chamber/nozzle assemblies of rockets consist of a series of coolant channels that run longitudinally along the chamber/nozzle wall, i.e., in the same direction as, although countercurrent to, the flow of the combustion gas within the nozzle. Channels running in this direction limit the efficiency of the heat transfer for several reasons. First, channels that extend the full length of the nozzle cause the coolant to undergo a relatively high pressure drop through the channel due to the length of the channel and the friction at the channel wall. Second, the coolant effect within any single coolant channel is stratified since coolant close to the hot gas bears a larger portion of the heat load than coolant further away from the hot gas. Third, the heat generated in the combustion chamber (generally the subsonic) section of the assembly is much greater than the heat generated in the skirt (generally the supersonic) section, and yet the total volumetric flow rate of coolant is the same in both sections, resulting in inefficient use of the coolant. With these inefficiencies, the temperature of the metal in the nozzle wall remains high and the useful life of the chamber/nozzle is relatively short. Furthermore, to accommodate coolant channels that are large enough to carry the volume of coolant needed to cool both sections of the chamber/nozzle, the jacket and hence the entire wall must be relatively thick which adds considerably to the weight of the assembly.